Just hours before Apple is set to report quarterly earnings, one top Internet analyst said the tech company's best days are behind it.

"The fear is that Apple is entering growth purgatory," senior analyst Toni Sacconaghi of Sanford C. Bernstein said Tuesday on CNBC's "Halftime Report." "It had the great last cycle with the iPhone 6. The fear is smartphones are in an increasingly mature marketplace, particularly at the high end. That's almost all of Apple's earnings and growth."

Shares of Apple dipped below Monday's close just past noon Tuesday, after Sacconaghi compared its single-digit growth projections to the double-digit expectations for competitors Amazon and Alphabet.

While Apple has gained share in the smartphone market, it faces challenges in continuing that trend, Sacconaghi said. Because the phone is such a large portion of Apple's revenue base, the company would be dependent on more far-fetched projects like its watch, TV and rumored self-driving car for growth, he said.

"You'd have to believe that the car would kick in, and drive revenues," Sacconaghi said. "Or that the watch would ultimately be something very big. But I think investors are worried about the next two-year period. Can there really be iPhone growth?"

It's been a tough week for Apple and its suppliers, with jittery markets watching data points on iPhone sales closely.

The comments are an about-face for Sacconaghi, who just two months ago told CNBC he believed iPhone sales could continue to grow in China and gain market share. Though he still believes the company will be a "moderate grower," Sacconaghi told CNBC that Wall Street has already factored Apple's decline into its math.

"I think its best days are behind it," Sacconaghi said. "The Street is discounting that Apple's free cash flow will decline in perpetuity."